1. Field of the Invention
The present invention relates to cutters for use on rotary drill bits for drilling subterranean formations and, more specifically, to a cutter including a superabrasive table including a cutting face and a side jacket over a supporting substrate, as well as rotary drill bits carrying such cutters.
2. State of the Art
Superabrasive materials, normally diamond, have been employed in cutting elements for rotary drill bits for decades. For about the past twenty-five years there has been widespread use of synthetic diamond cutters, specifically in the form of polycrystalline diamond compacts. Polycrystalline diamond compact cutters, commonly known as PDCs, have been commercially available for over 20 years. PDCs may be self-supporting, or may comprise a diamond xe2x80x9ctablexe2x80x9d bonded during formation to a supporting substrate. A diamond table/substrate cutter structure is formed by stacking into a cell layers of fine diamond crystals (100 microns or less) and metal catalyst powder, alternating with wafer-like metal substrates of cemented tungsten carbide or other suitable materials. In some cases, the catalyst material may be incorporated in the substrate in addition to or in lieu of using a powder catalyst intermixed with the diamond crystals. A loaded receptacle is subsequently placed in an ultra-high temperature (typically 1450-1600xc2x0 C.) ultra-high pressure (typically 50-70 kilobar) diamond press, wherein the diamond crystals, stimulated by the catalytic effect of the metal powder, bond to each other and to the substrate material. The spaces in the diamond table between the diamond to diamond bonds are filled with residual metal catalyst. A so-called thermally stable PDC product (commonly termed as xe2x80x9cTSPxe2x80x9d) may be formed by leaching out the metal in the diamond table. Alternatively, silicon, which possesses a coefficient of thermal expansion similar to that of diamond, may be used to bond diamond particles to produce a Si-bonded TSP. TSPs are capable of enduring higher temperatures (on the order of 1200xc2x0 C.) without degradation in comparison to normal PDCs, which experience thermal degradation upon exposure to temperatures of about 750-800xc2x0 C.
While PDC and TSP cutters employed in rotary drag bits for earth boring have achieved major advances in obtainable rate of penetration while drilling and in greatly expanding the types of formations suitable for drilling with diamond bits at economically viable cost, the diamond table/substrate configurations of state of the art cutters, typically employing substantially planar superabrasive tables having a variety of interface configurations with a supporting substrate, leave something to be desired.
First, bending, attributable to the loading of the cutting element by the formation, may cause fracture or even delamination of the diamond table from the substrate. It is believed that such degradation of the cutting element is due, at least in part, to lack of sufficient stiffness of the cutting element so that when encountering the formation the diamond table actually flexes due to lack of sufficient rigidity or stiffness. As diamond has an extremely low strain to failure (diamond cannot tolerate large values of absolute strain), only a small amount of flex can initiate fracture. In addition, fracture may also be initiated in the highly stressed carbide substrate when cutting loads are applied to the cutting element, as the carbide is stressed in tension during cooling after the previously-described fabrication process due to the difference in coefficients of thermal expansion between the diamond and the substrate material.
A second limitation of PDCs is due to excessive buildup of heat due to frictional forces generated during the cutting process. While the superabrasive material of the cutting element table has an extremely high thermal conductivity (on the order of 400 to over 600 watts/meter Kelvin) and the substrate has a relatively high thermal conductivity (on the order of 100 watts/meter Kelvin), the bit body, typically steel or WC matrix, has a far lower thermal conductivity (on the order of 30 watts/meter Kelvin). As the cutting element wears and the point of contact with the formation becomes an ever-wider wear flat, the cutting element is subjected to higher cutting energies and the substrate becomes ever-smaller, limiting and actually reducing the potential rate of heat transfer. The heat buildup causes overheating of the cutting element and accelerated wear of the diamond table and supporting substrate. In xe2x80x9cdullxe2x80x9d or used bits, such excessive heating is often manifested on the WC substrate behind the diamond table by the phenomenon of xe2x80x9cheat checkingxe2x80x9d, which comprises vertically running fractures in a checkerboard pattern.
It has been proposed to enhance the stiffness of superabrasive cutting elements by providing the superabrasive table with a linearly-extending portion of enhanced thickness. Such a configuration provides additional stiffness for the cutting structure, and also beneficially increases compressive stresses in the superabrasive material table while lowering tensile stresses in the supporting substrate. A number of variations of this approach are described in co-pending U.S. Pat. No. 5,435,403 to Gordon A. Tibbitts, assigned to the assignee of the present invention and incorporated herein by this reference.
It has also been proposed to provide superabrasive cutters with diamond tables including one or more struts or other protrusions of superabrasive material extending rearwardly into the substrate to enhance stiffness of the table, as well as, or alternatively, to enhance heat transfer from the cutting edge and cutting face of the diamond table. U.S. Pat. No. 5,590,729 to Cooley et al., assigned to the assignee of the present invention and incorporated herein by this reference, discloses a variety of such cutters.
Yet another advance in the art was the recognition that cutters in different locations on drill bits experience loading of different magnitudes and types during a drilling operation, and that cutters might be designed and selected to best accommodate loading at the different locations. U.S. Pat. No. 5,605,198 to Tibbitts et al., assigned to the assignee of the present invention and incorporated herein by this reference, discloses such design and selective placement of cutters.
U.S. Pat. No. 5,590,727 to Tank discloses several cutter configurations employing a stepped interface between superabrasive material and a supporting substrate, the superabrasive material extending down an exterior side of the cutter.
U.S. Pat. No. 5,667,028 to Truax et al. discloses a variety of cutter configurations employing so-called xe2x80x9csecondaryxe2x80x9d PDC cutting surfaces placed on the side of the substrate in spaced relationship to the PDC diamond table, the secondary cutting surfaces purportedly reducing the rate of erosion of the substrate material during drilling.
However, despite the above-referenced developments, significant shortcomings are still exhibited by conventional cutters in certain situations. For example, erosion or abrasion of the cutter substrate immediately to the rear of the superabrasive table results in the beneficial formation of a protruding xe2x80x9clipxe2x80x9d of superabrasive material and conventional thought is to the effect that the presence of such a lip facilitates the cutting action of the cutter. However, the inventors herein have recognized that the well-known phenomenon of so-called bit xe2x80x9cwhirlxe2x80x9d, wherein a bit rotates or precesses in the borehole counter to the direction of bit rotation by the drill string or downhole motor, may result in the superabrasive lip xe2x80x9ccatchingxe2x80x9d on the uncut formation of the borehole bottom or wall so that the superabrasive table is placed in tension, precipitating delamination of the table from the substrate. Further, the inventors have recognized that in more ductile, elastic formations, subsequent to shearing of formation material by the superabrasive table of a cutter, the still-uncut formation adjacent the cutter rebounds and contacts the substrate to the rear of the table. This phenomenon, occurring on a continuing basis as the bit rotates, results in the aforementioned heat checking of the substrate and attendant breakdown in physical support for the superabrasive table.
Accordingly, there remains a significant need in the art for improvements in cutter integrity, impact resistance and heat transfer capabilities.
The present invention includes a cutter comprising a superabrasive volume including a cutting face portion extending transversely across at least a portion of a leading end of a supporting substrate and a contiguous jacket portion extending rearwardly over the supporting substrate along a portion of its side periphery comprising a surface of revolution. Interfaces between the respective superabrasive volume cutting face and jacket portions and adjacent exterior surfaces of the supporting substrate are each irregular. More specifically, the leading face of the substrate may include one or more grooves extending toward the jacket at least partially across the leading face from the side of the substrate opposite the jacket, the superabrasive material extending into the grooves. Similarly, the side periphery of the substrate defining the location of the jacket may be grooved at one or more adjacent locations with substantially axially-oriented grooves extending rearwardly from the leading end of the substrate to a position closer to the trailing end of the substrate. Substantially axially-oriented ridges intermediate the side periphery grooves may not extend radially outwardly to a full radius of the adjacent substrate portion so that the superabrasive material of the jacket extends not only into the circumferential grooves but also circumferentially therebetween, providing a continuous, arcuate superabrasive surface having substantially the same exterior radius as that of the substrate portion adjacent the jacket. Alternatively, if the ridges extend to the full radius of the substrate portion adjacent the jacket, the jacket may comprise mutually adjacent but separate ribs of superabrasive material rather than a continuous surface.
The asymmetrical design of the inventive cutter provides a significant substrate surface area for brazing of the cutters into pockets on the face of a bit while protecting the carbide material of the cutter substrate on the bottom side of the cutter (as the cutter is normally oriented on the bit), as well as the interface between the superabrasive table and the substrate.
Rotary drill bits for subterranean drilling bearing cutters according to the present invention are also included within the scope of the invention.